Hermetically sealed resistor

ABSTRACT

A resistance film is formed on the inner surface of tube of dielectric material. End terminals interfused with the tube provide electrical continuity between the resistance film and metallic end caps closing the ends of tube, which end caps are soldered in place to provide heretic seals.

United States Patent Brandi Feb. 15, 1972 [54] HERMETICALLY SEALEDRESISTOR 2,786,925 3/1957 Kahan ..338/329 X 2,792,620 5/1957 Kohring........338/258 X [m 3.73% Brand" 3,249,904 5/1966 Karalis ..338/308 x [22]Filed: June 1, 1970 Primary ExaminerE. A. Goldberg An -RdlhJ.J k 21Appl. No.: 41,810 op [52] 11.8. C1 ..338/329, 338/258, 338/331, ABSTRACT338/332 A resistance film is formed on the inner surface of tube of [5]]Int. Cl .1101! l/l4 dielectric materiaL End terminals imcrfused with htube [53] Field of Search 338/258 provide electrical continuity betweenthe resistance film and 338/332, metallic end caps closing the ends oftube, which end caps are soldered in place to provide heretic seals.[56] References Cited 2 Claims, 4 Drawing Figures UNITED STATES PATENTS1,835,582 12/1931 Allen ..338/260 X 2,412,462 12/1946 Marsten ..338/258X /6 l2 l0 l5 I 1 0; 50 0 I l IIERMETICALLY SEALED RESISTOR BACKGROUNDOF THE INVENTION Electrical resistors comprising a metallic filmdeposited on the inner surface of a tube and provided with metallic endcaps are well known. In the case of a ceramic tube, end terminals areinterfused with the tube and serve to provide electri'c'al continuitybetween the resistance film and the end caps. Heretofore, silver platedend caps have been press-fitted over the end terminals. After the tubehas been heated and cooled, the silver alloy joins together thecontiguous walls of the end caps and the underlying portions of the endterminals. In many cases, such joint provides a hermetic seal but oftenthe joint is defective as pinholes develop in the silver seal. Suchpinholes develop under the end cap and there is no certain way oflocating or eliminating them. Thus, the seal is either perfect ofdefective when first made, resulting in a costly rejection of re sistorswhich have proceeded to the final step in the manufacture thereof.

The present invention eliminates the present uncertainty in themanufacture of hermetically sealed resistors to meet militaryspecifications. The hermetic seal can readily be tested for leakage anddefective seals are visually detectable and easily corrected.

ABSTRACT OF THE INVENTIon End tenninals are interfused with a ceramictube having a resistance film deposited on the inner surface thereof.The end terminals, which extend from the inner surface of the tube tothe outer surface thereof, provide electrical continuity between thefilm and metal end caps press-fitted over the end terminals which havebeen coated with a solder alloy. Upon heating and then cooling the tube,the contiguous surfaces of the end caps and the end terminals are joinedtogether. In accordance with one embodiment of this invention, theexterior portions of the end terminals project beyond the edges of theend caps and a bridge of solder alloy is formed between such edges andthe projecting portions of the associated end terminals therebyresulting in a visible solder joint of substantial mass. In anotherembodiment of the invention, outer metallic bands are interfused withthe ceramic tube, which bands are proximate to, but spaced from, theexterior portions of the end terminals. These bands project beyond theedges of the end caps and the solder alloy bridge is formed between theedges of the end caps and the projecting portions of the associatedbands. In both embodiments of the invention, the bridging solder jointhas a substantial thickness at the critical area,

namely, the circumferential region defining the area of contact betweenthe end cap and the underlying metallic band, or the underlying exteriorportion of the end terminal.

An object of this invention is the provision of an arrangement formaking a hermetically sealed resistor and a resistor made by suchmethod.

An object of this invention is the provision of a hermetically sealedresistor of the class having a resistance film formed on the interiorsurface of a tube.

An object of this invention is the provision of an improved arrangementfor hermetically sealing a resistor having metal end caps closing theends of a ceramic tube carrying an internal resistance film, a solderalloy bridging the edges of the end caps and metallic members interfusedwith the tube.

The above-stated and other objects and advantages of the invention willbecome apparent from the following description when taken with theaccompanying drawings. It will be understood, however, that the drawingsare for purposes of illustration and are not to be construed as definingthe scope or limits of the invention, reference being had for the latterpurpose to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings wherein like referencecharacters denote like parts in the several views:

FIG. 1 is a side elevational view of a resistor made in accordance withthis invention.

FIG. 2 is an end elevational view thereof;

FIG. 3 is a fragmentary cross-sectional view taken along the line 3-3 ofFIG. '1 and drawn to an enlarged scale; and

FIG. 4 is a similar cross-sectional view showing another embodiment ofthe invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. 1-3, a resistormade in accordance with this invention comprises a tube 10 made ofsuitable dielectric material, preferably a ceramic, and having aresistance film I1, formed on the inner surface thereof. The ends of thetube are sealed by bronze, silver-plated end caps 12 and 12', which capshave lead wires 13 and 13' welded thereto.

Referring particularly to FIG. 3, the end portion of the tube I0 isfirst coated with a metal paste or frit, preferably a silver frit,thereby forming an end terminal 14. A silver frit also is applied to theouter surface of the tube to form the band l5, which band is spaced fromthe end terminal 14. The other end of the tube is similarly treated toprovide a corresponding end terminal and outer band. The unit is firedat an elevated temperature, whereby the silver frit coatings areinterfused with the tube. Thereafter, the resistance film is formed bydepositing a metal ontothe inner surface of the tube and over the innerportions .of the end terminals. Preferably, a nickelchrome alloy isevaporated onto the tube surface by methods well known in this art.Next, the ohmic resistance of the film is adjusted to a desired valueas, for example, by grinding the illustrated helical groove through thefilm. The band 15 and the outer surface of the end terminal 14 areplated with copper after which the silver plated end cap 12 is pressedover the band, it being noted that the outer band 15 extends beyond theend of the end cap.

With the twoend caps in place the unit is evacuated and then filled withan inert gas at about 30 pounds per square inch above atmosphericpressure, after which the pressure is released to 14.7- p .s.i. toestablish equilibrium with the atmospheric pressure. A soldering flux isapplied to the exposed portions of the bands 15 after which the unit isrolled on the surface of a hotplate and into a mass of solder alloycarried by the hotplate and maintained in a liquid state. The solderalloy bridges the edges of the end caps and the associated outer bandsto form a readily visible joint 16. Also, the solder alloy flows betweenthe contiguous surfaces of the end caps, the outer bands and the endterminals.

The described arrangement for closing the ends of the tube creates ahermetic seal which, under test has an average leakage rate of l X 10cubic centimeters per second, whereby the resistor meets the seal testfor hermetically sealed resistors under military specifications.Furthennore, when the finished resistor is subjected to the leak test anincomplete hermetic seal is detected by the formation of minute bubbleson the joint 16. In a majority of cases, such resistor can be salvagedby repeating the above-described soldering procedure. The completedresistor may be externally insulated by encapsulating the body of theresistor in a nonflammable epoxy resin or by means of a plastic shrinktube.

A modification of the sealing arrangement is shown in FIG. 4. Here, theouter portion of the end terminal 17 extends beyond the end cap 12, withthe solder joint l8 bridging the edge of the cap and the extendedsurface of the end terminal. The end terminall7 is formed by applying asilver frit to the tube surfaces by means of a brush, thereby to formthe inner portion of the terminal of a relatively short axial length (inorder not to undesirably reduce the effective axial length of theresistance film) while, at the same time, forming the outer portion ofthe terminal of a desired, longer axial length. In the embodiment of theinvention shown in FIG. 3, the ends of the tube may be dipped in thesilver frit to form the end terminals which serve primarily to establishelectrical continuity between the resistance film and the end capsHaving now describedthe invention what I desire to protect by LettersPatent is set forth in the following claims.

1. A resistor comprising,

a. a tube of dielectric material,

-3 4 b. metallic end terminals on the ends of the tube and interendterminal and the edge of each end cap terminating at fused therewith,each terminal including an interior pora point intermediate the ends ofthe proximate one of said tion on the inner wall of the tube and anexterior portion metallic bands, and on the outer wall of the tube, f. asolder alloy joining together the contiguous surfaces of c. a metallicresistance film on the inner wall of the tube and 5 each of the end pthe associated metallic nd and th overlying at least a portion of theinterior portions of the exterior Portion of the end terminal saidSolder also said and termi l bridging the edges of the end caps and theprojecting pord. a pair of metallic bands on the outer wall of saidtube, tions of the associated metallic bands- 2. The invention asrecited in claim 1, wherein the tube is made of a ceramic, and whereinthe end caps are silver-plated and have connection leads welded thereto.

each band being interfused with the tube and spaced from the exteriorportion of one of the said end terminals,

e. metallic end caps closing the ends of the tube, each end capextendingover the exterior portion of the associated

1. A resistor comprising, a. a tube of dielectric material, b. metallicend terminals on the ends of the tube and interfused therewith, eachterminal including an interior portion on the inner wall of the tube andan exterior portion on the outer wall of the tube, c. a metallicresistance film on the inner wall of the tube and overlying at least aportion of the interior portions of the said end terminals, d. a pair ofmetallic bands on the outer wall of said tube, each band beinginterfused with the tube and spaced from the exterior portion of one ofthe said end terminals, e. metallic end caps closing the ends of thetube, each end cap extending over the exterior portion of the associatedend terminal and the edge of each end cap terminating at a pointintermediate the ends of the proximate one of said metallic bands, andf. a solder alloy joining together the contiguous surfaces of each ofthe end caps, the associated metallic band and the exterior portion ofthe end terminal, said solder alloy also bridging the edges of the endcaps and the projecting portions of the associated metallic bands. 2.The invention as recited in claim 1, wherein the tube is made of aceramic, and wherein the end caps are silver-plated and have connectionleads welded thereto.